[As8W48O184](40-), a new crown-shaped heteropolyanion: Electrochemistry and electrocatalytic properties towards reduction of nitrite

The electrochemical behavior of the new crown-shaped heteropolyanion [As8W48O184](40-) was investigated. Compared to the analogous complex [P8W48O184](40-) [As8W48O184](40-) displayed a close similar behavior. As previously reported for [P8W48O184](40-), [As8W48O184](40-) is found to be stable in a large range of pH, roughly from 0 to 8 and found to be slightly more soluble than [P8W48O184](40-). Comparison between the cyclic voltammetry curves of the two complexes reveals that the potentials of the redox waves of [As8W48O184](40-) are slightly shifted towards the positive values. Contrary to the high dependence of [P8W48O184](40-) to buffer capacity of the supporting electrolyte, interestingly, [As8W48O184](40-) displays less sensitivity. The numerous possibilities for chemical applications opened up by the electrochemistry of this new complex are exemplified here by the good linear relationship between the peak potentials of the redox waves as a function of pH and, by the electrocatalytic reduction of nitrite. (C) 2010 Elsevier Ltd. All rights reserved.status: publishe

Attempts to immobilize catalytically active substituted-heteropolytungstates in multilayer film of charged polyelectrolyte poly(allylamine hydrochloride)

A series of transition metal substituted polyoxometalates (POMs) have been attached to carbon electrodes in multilayer films of poly(allylamine hydrochloride) (PAH). These include Wells-Dawson-derived sandwich-type Fe-4(III)(H2O)(2)(As2W15O56)(2)(12-)center dot CO4II(H2O)(2)(As2W15O56)(2)(16-) and Mn-4(II)(H2O)(2)(As2W15O56)(2)(16-) and Wells-Dawson mono-substituted-type alpha(1)-[(P2W17VO62)-O-IV](8-) and alpha(2)-[P2W17O61(OH)(2)Fe-III](7-). The modified electrodes were characterized by cyclic voltammetry and UV-Vis spectroscopy. The experimental observations indicate the degradation of the substituted-POMs during their incorporation in the PAH multilayer film, probably as a result of electrostatic repulsions between the positively-charged metal cation substitute (Fe-III Mn-II or V-IV) of the POM and the positively-charged PAH. The degradation mostly results in the formation of lacunary-type POMs. Although the encapsulation does not lead to the entire preservation of the substituted-POMs, the substituted-POMs/PAH films can be used as pH sensors and as efficient electrocatalysts for the reduction of dioxygen, hydrogen peroxide and the oxidation of the NADH coenzyme.status: publishe

Encapsulated-polyoxometalates in surfactant/silica gel hybrid films: Electrochemical behavior and main characteristics

Immobilization of various polyoxometalates (POMs) of different sizes and charges in surfactant/silica gel hybrid films is investigated. A change in the electrochemical behavior is noticed between dissolved and immobilized POMs. This change is attributed to the microenvironment effect that POM clusters experience inside the films. The presence of surfactant Triton X-405 in the matrix is essential for these observations. Results from SEM showed rough and porous morphology for the films prepared with surfactant, while smooth and fractured film structure is observed in absence of surfactant. For POMs such as [P2W18O62](6-) and [SiW12O40](4-), the first two one-electron redox waves merged into one two-electron redox wave. However, for other POMs such as [PW12O40](3-). [H4AsW18O62](7-) (P2W15MO2VO62)-O-IV](7-). of alpha(1)-P2W17O61FeIII](7-), of alpha(2)-[P2W17VO62](8-) and [P8W48O184](40-), the microenvironment effect results only in a shift of the potentials towards the negative values. The magnitude of the shift in potential is shown to decrease with increasing the POM's charge. Results from Particle size analysis showed that Triton X-405 influences the POM's particle size and with increasing the POM's charge, the POM's particle size in presence of Triton X-405 decreases. The latter is attributed to formation of micelles POM-Triton X-405, which consequently create the microenvironment effect for the immobilized POMs. The main characteristics of the surfactant/silica gel immobilized POM films are explored. The influence of the scan rate demonstrates that the redox behavior of the surfactant/silica gel immobilized composites is fast and involves surface-confined electron transfer processes. The study of these films in different proton concentrations showed that they are stable at low pH, and to obtain a better electron transfer, they have to be stored in the reference medium of 0.5 M H2SO4. (C) 2010 Elsevier Ltd. All rights reserved.status: publishe

Manganous heteropolytungstates. Synthesis and heteroatom effects in Wells–Dawson-derived sandwich complexes

International audienc

Tuning the Photochromic Properties of Molybdenum Bisphosphonate Polyoxometalates

International audienceSeven hybrid organic−inorganic bisphosphonate molybdenum(VI) polyoxometalate complexes with the general formula [(Mo3O8)4(O3PC(CmH2mNRR′R″)(O)PO3)4]8− (m = 3; R, R′, and R″ = H or CH3) and [(Mo3O8)2(O)- (O3PC(CmH2mNRR′R″)(O)PO3)2]6− (m = 3 or 4; R, R′, and R″ = H or CH3) have been synthesized and their structures solved using single-crystal X-ray diffraction. These compounds are made of a {Mo12} or a {Mo6} inorganic core functionalized by various alkylammonium bisphosphonates, with these ligands differing by the length of their alkyl chains and the number of methyl groups grafted on the N atom. The nature of the counter-cations (Na+, K+, Rb+, Cs+, and/or NH4 +) constituting these materials has also been modulated. 31P NMR spectroscopic studies in aqueous media have shown that all the dodecanuclear complexes reported here are stable in solution, whereas for the hexanuclear compounds, a dynamic equilibrium between two isomers has been evidenced, and the corresponding standard thermodynamic parameters determined for one of them. The electrochemical properties of six representative compounds of this family have been investigated. It has been found that the Mo6+/Mo5+ reduction potential is similar for all the polyoxometalates studied. Besides, it is shown that electrochemical cycling is an efficient method for the deposition of these compounds on a surface. The photochromic properties of all the complexes reported herein have been studied in the solid state. Under irradiation in the near ultraviolet (UV), the {Mo12} systems shift from white to reddish-brown, while the {Mo6} compounds develop a purple coloration. The coloration kinetics has been systematically quantified and the optical band gaps, the salient coloration kinetic parameters and the coloration kinetic half-life times have been determined. This has evidenced that several of these materials develop very strong and rapid UV-induced color changes, with remarkable coloration contrasts. Finally, the optical properties of these systems are discussed in light of several salient parameters as the POM topology, the nature of the grafted bisphosphonate ligand, and the design of the hydrogenbonding network at the organic−inorganic interface